Electron paramagnetic resonance (EPR) imaging using methods analogous to those employed in nuclear magnetic resonance (NMR) imaging but at much higher nutation frequencies is known. However, in view of the strong magnetic field gradients necessary to achieve good spatial resolution, EPR imaging has so far been restricted to small samples.
It is also known that if nuclei of a solvent in which paramagnetic material is dissolved are excited to nuclear magnetic resonance and the NMR resonance is observed, a dramatic enhancement of the observed NMR signal may be obtained if the paramagnetic material is simultaneously excited to EPR resonance. This phenomenon, known as Proton-Electron Double-Resonance Imaging (PEDRI), may be utilized to obtain image information regarding the spatial distribution of paramagnetic material in solution, and our U.S. Pat. No. 4,891,593 (the contents of which are incorporated herein by this reference to it) discloses a method of obtaining image information representing the distribution of paramagnetic material in solution which comprises the steps of applying radio-frequency radiation to excite EPR resonance in the solute and obtaining an NMR image signal of selected nuclei preferably protons) of the solvent, the signal from those selected nuclei which interact with electrons excited by the rf radiation being enhanced.
As will be understood, the solution to which this method is applied may be inhomogenously distributed in a sample, of living tissue, for example, and the enhanced parts of the obtained NMR image then represent the spatial distribution of the paramagnetic material within the tissue sample.
The NMR signal enhancement, which occurs in regions of the sample where paramagnetic material is present and influences the NMR proton relaxation rate, and which appears in the final image as a locally increased intensity of the image, may be defined as E=A.sub.z /A.sub.o, where A.sub.z and A.sub.o are the NMR signals with and without EPR irradiation. It is known that E depends on the concentration of the paramagnetic solute and on the square of the EPR irradiation radiofrequency magnetic field which is in turn proportional to the power of the EPR irradiation. If the conductivity of the sample is assumed to be constant, the power of the EPR irradiation required to produce a given value of E can be shown to be proportional to the square of the EPR irradiation frequency.
Since a proportion of the applied radiofrequency power is always absorbed by the sample, it is desirable to minimize the applied power while maintaining a detectable enhancement. Thus it is desirable to use as low a value of the polarizing magnetic field B.sub.o as possible, since the EPR frequency is proportional to the B.sub.o NMR magnetic field. Considering the signal-to-noise ratio (SNR) of the NMR image, however, it is known that the SNR decreases rapidly with decreasing B.sub.o. Thus the value of B.sub.o should be maximized to optimize the SNR, which will in turn improve the sensitivity of detection of the paramagnetic species. Thus there are, apparently, conflicting requirements applying to selection of the value of B.sub.o.
In our pending international patent application WO 92/04640 (the contents of which are incorporated herein by this reference to it) we have described how these apparently conflicting requirements as to the magnitude of the NMR magnetic field B.sub.o can both be satisfied and the apparent conflict resolved, by obtaining the NMR signal by use of the NMR method known as magnetic field cycling NMR. The magnetic field cycling NMR method, which is known as a method of studying NMR relaxation and other phenomena at extremely low field strengths (F. Noack, Prog. NMR Spectrosc. 18. 171(1986)), has three distinct periods during each of which B.sub.o may have a different value: polarization at B.sub.o.sup.p (high field), evolution at B.sub.o.sup.e (low field) and detection at B.sub.o.sup.d (intermediate or high field); and, as disclosed in the said international application, the EPR irradiation magnetic field for exciting EPR resonance, and thereby enhancing the NMR signals from nuclei which interact with the excited electrons of the paramagnetic material, is applied only during the evolution period of each cycle, when the B.sub.o.sup.e field value is low and the required frequency and power of the EPR irradiation are correspondingly low.
This method, combining the PEDRI double-resonance method with the field-cycling method, can effectively minimise the total of RF energy which the object under examination will absorb during each NMR excitation; but in using the method to produce a PEDRI-NMR image having 32.times.32 pixels at least 32 periods of EPR irradiation are required and each of these periods of RF irradiation contributes to the total energy absorbed by the object under examination. It is always desirable to obtain as high an enhancement of the NMR signal as possible, in order to improve the sensitivity of PEDRI for the detection of low concentrations of free radicals, so that in general as high an RF power as is tolerable will be used. Thus, in many cases, the ultimate sensitivity will be limited by the permissible applied power level. One way of reducing the average applied power is simply to extend the time duration of each imaging pulse sequence thus decreasing the duty cycle of the RF irradiation, but this may well increase the total imaging time to beyond acceptable limits, particularly if images with 128.times.128 or 256.times.256 pixels are being collected.